At about 20 sites in the Raton basin of Colorado and New Mexico and at least 10 sites in Wyoming, Montana, and western Canada, a pair of claystone units, an iridium (Ir) abundance anomaly, and a concentration of shock-metamorphosed minerals mark the palynological Cretaceous/Tertiary (K/T) boundary. The lower unit, the K/T boundary claystone bed, is generally 1 to 2 cm thick; the upper unit, the K/T boundary impact layer is, on average, 5 mm thick. This couplet is overlain by a coal bed 1 to 16 cm thick.

The boundary claystone in the Raton basin consists mainly of kaolinite and...